A multi fuel engine refers generically to any type of engine, boiler, heater or other fuel-burning device which is designed to burn multiple types of fuels in its operation. Multi fuel engines have application in diverse areas to meet particular operational needs in the operating environment. For example, a common use of multi fuel engines is in military vehicles so that vehicles in various deployment locations may run a wide range of alternative fuels such as gasoline, diesel or aviation fuel. In combat settings, for example, enemy action or unit isolation may limit the available fuel supply and personnel may need to resort the type of fuel available for usage from enemy and civilian sources. Multi fuel engines are also desirable where cheaper fuel sources, such as natural gas, are available, but an alternative or secondary fuel, such as diesel fuel, is needed for performance reasons (e.g., faster reaction to short term load demand), as a backup in the event of an interruption in the supply of the primary fuel source, or for other operational or engine performance conditions.
A multi fuel engine typically operates with a specified mixture of the available fuels. Where a liquid-only fuel mixture is specified, a liquid fuel, such as diesel fuel, gasoline or other liquid hydrocarbon fuel, is injected directly into an engine cylinder or a pre-combustion chamber as the sole source of energy during combustion. When a liquid and gaseous fuel mixture is specified, a gaseous fuel, such as natural gas, methane, hexane, pentane or any other appropriate gaseous hydrocarbon fuel, may be mixed with air in an intake port of a cylinder and a small amount or pilot amount of liquid fuel, such as diesel fuel, is injected into the cylinder or the pre-combustion chamber in an amount according to a specified substitution ratio in order to ignite the mixture of air and gaseous fuel.
A typical engine speed controller has one controller that acts on speed error to set a fuel rate. For engines that may run on multiple fuels, it is required to set multiple fuel rates based on the fuel fraction or desired ratio of fuels. For example, it may be desired to run a multi fuel engine on a mixture of 80% natural gas and 20% diesel. However, typical speed controllers (usually proportional-integral controllers, commonly called PI controllers) may only set a fuel rate for a single fuel. The normal way to deal with a multi fuel engine is to have each PI controller set an individual fuel rate for the corresponding fuel while ignoring the fact that there are other fuels supplying power to the engine. The fuel rates are set as if the other fuels do not exist. After the individual fuel rates are set by the PI controllers, a complicated switching strategy manages the multiple fuel rates, and selects a composite fuel flow based on the specified fuel mixture. The selected composite fuel flow accounts for the availability of the other fuels. If a specific fraction of fuel is desired, such as the 80% natural gas, 20% diesel fuel mixture discussed above, the switching strategy will output multiple fuel flow rates. In this case, separate control signals will be output to the flow control devices for natural gas and diesel fuel to create the fuel flows of each fuel that are necessary for the composite fuel flow. The disadvantages of this type of control structure include the significant amount of design time and effort required for multiple PI controllers and the complexity of the switching strategy to ensure that the overall design is robust and responsive to changes in the input power requirements.
The fuel properties for the fuel may have to be manually input each time an engine tank has to be refilled. A quality of the fuel being used in the engine and the fuel tested in the lab may be different. Also, the quality of the fuel may change after operating the engine for a certain time duration. Under such circumstances, the fuel flow rate determined based on the input fuel properties may not be accurate. In view of these conditions, a need exists for an improved multi fuel engine control strategy that simplifies the process for determining the fuel flow rates for the various fuels available to provide power to the engine. A further need exists for the multi fuel engine control strategy to adjust or determine the fuel flow rate to provide the necessary power to the engine by considering the changes in fuel quality of the fuel.